The present invention relates to a process for the separation of diacetyl from methyl methacrylate, which separation has particular value in the manufacture of monomeric methyl methacrylate by reaction of acetone cyanohydrin with sulphuric acid in order to form methacrylamide, which is then esterified with methanol.
With such a manufacturing process, the monomeric methyl methacrylate is obtained with about 1 to 5 ppm of biacetyl (2,3-butanedione) as impurity responsible for the yellowing of the poly(methyl methacrylate). In order to avoid this problem of yellowing of the polymer formed, the process must be performed such that the biacetyl content in the monomeric methyl methacrylate is less than 0.1 ppm.
In accordance with European patent EP-B-0,206,230, it is proposed to add to the esterification reaction mixture an effective amount of an aromatic ortho-diamine, such as ortho-phenylenediamine, in the presence of an acid catalyst (for example sulphuric acid), which is present in an amount of 0.1 to 65% by weight of the reaction mixture. The aromatic orthodiamine may also be added after separation of the finished monomer, with addition of a suitable amount of inorganic acid.
Such a process allows for satisfactory removal of the biacetyl; however, it has the drawbacks of having to use aromatic ortho-diamines which are expensive and toxic compounds and which may result in a decrease in the stability of the monomeric methyl methacrylate, on account of the presence in the latter of the product of reaction of the biacetyl and the aromatic ortho-diamine. This may lead to frequent encrustation of the distillation column heating pipes.